Pasciak Alexander S, Bourgeois Austin C, Bradley Yong C
Department of Radiology, University of Tennessee Graduate School of Medicine, Knoxville, Tennessee School of Medicine, Johns Hopkins Hospital, Baltimore, Maryland; and
Department of Radiology, University of Tennessee Graduate School of Medicine, Knoxville, Tennessee Department of Radiology, Medical University of South Carolina, Charleston, South Carolina.
J Nucl Med. 2016 Jul;57(7):1020-6. doi: 10.2967/jnumed.115.163444. Epub 2016 Feb 18.
Differences in maximum tolerable absorbed dose to normal liver between (90)Y radioembolization and external-beam radiation therapy have been explained by citing differences in absorbed-dose heterogeneity at the microscopic level. We investigated microscopic absorbed-dose heterogeneity in radioembolization as a function of the number of microspheres per unit volume in tumor. The goal was to determine what effect the number of microspheres may have, if any, on tumor control in (90)Y radioembolization.
(90)Y PET/CT data were combined with microscopic probability-density functions describing microsphere clustering to provide realistic simulation using Monte Carlo modeling on both a macroscopic and a microscopic level. A complete microdosimetric analysis using 100-μm voxels was performed on the basis of (90)Y PET/CT data from 19 tumors treated using radioembolization. Simulations were performed with average tumor microsphere-number densities from 200 to 70,000 spheres/mL. Monte Carlo simulations of each tumor and number density were repeated 20 times to establish SE. A 2-way balanced ANOVA was used to determine whether differences in microsphere-number density affected common tumor-dose metrics.
Decreasing the microsphere-number density resulted in a decrease in D70, the minimum dose to 70% of the tumor. The slope of the dose-volume histogram also decreased with decreasing microsphere-number density in all tumors. Compared with a density of 50,000 spheres/mL, decreases in D70 were statistically significant below 20,000 spheres/mL. However, these differences are unlikely to have clinical significance until the density decreases to below 5,000 spheres/mL. Although D70 was decreased at a low microsphere-number density, one can compensate for decreases by an increase in the average tumor-absorbed dose, that is, by increasing the radioembolization treatment dose.
Differences in microsphere-number density may have an effect on microscopic tumor absorbed-dose inhomogeneity. These results begin to explain differences in treatment planning strategies between glass and resin radioembolization devices.
通过引用微观层面吸收剂量异质性的差异，解释了钇-90放射性栓塞与外照射放疗对正常肝脏的最大耐受吸收剂量的差异。我们研究了放射性栓塞中微观吸收剂量异质性与肿瘤中每单位体积微球数量的函数关系。目的是确定微球数量对钇-90放射性栓塞中肿瘤控制可能产生的影响（如果有）。
将钇-90正电子发射断层扫描/计算机断层扫描（PET/CT）数据与描述微球聚集的微观概率密度函数相结合，以在宏观和微观层面上使用蒙特卡罗建模进行真实模拟。基于19例接受放射性栓塞治疗的肿瘤的钇-90 PET/CT数据，对100微米的体素进行了完整的微剂量分析。模拟的平均肿瘤微球数密度为200至70000个/毫升。对每个肿瘤和数密度进行20次蒙特卡罗模拟以确定标准误。使用双向平衡方差分析来确定微球数密度的差异是否影响常见的肿瘤剂量指标。
降低微球数密度导致D70（肿瘤70%体积所接受的最小剂量）降低。所有肿瘤的剂量体积直方图斜率也随着微球数密度的降低而减小。与50000个/毫升的密度相比，当密度低于20000个/毫升时，D70的降低具有统计学意义。然而，直到密度降至5000个/毫升以下，这些差异才可能具有临床意义。尽管在低微球数密度下D70降低，但可以通过增加平均肿瘤吸收剂量（即增加放射性栓塞治疗剂量）来弥补这种降低。
微球数密度的差异可能会影响肿瘤微观吸收剂量的不均匀性。这些结果开始解释玻璃和树脂放射性栓塞装置在治疗计划策略上的差异。
